The invention relates to a device for modulating a first rotational motion of an input shaft to a second, different from the first, rotational motion of an output shaft in textile machines, comprising                a fixed complementary cam provided with at least two cam profiles;        at least one cam follower, which is linked to the input and the output shafts and which is provided with at least two contact rollers, each following a different cam profile, because of which the said cam followers are performing a rocking motion.        
In textile machines, i.e. weaving machines, Jacquard machines, knitting machines, etc. quite a number of elements are moving in accordance with an oscillating, reciprocating or an irregular rotational motion in connection with a rotating, most of the time continuously rotating main shaft.
Some examples are:                beating up weft yams by the weaving reed in a weaving machine;        driving the knifes of a Jacquard machine;        driving the rapiers of a rapier weaving machine;        driving a weaving frame by means of eccentrics, cam discs or an electronic dobby;        forming the shed for the selvedge in a weaving machine;        etc.        
In order to convert the continuous rotational motion of the main shaft into a reciprocating motion, it is known to use a separate motor to perform the motion desired in synchronization with the main shaft. Using separate motors, controlled in synchronization, however, is expensive and also involves risks in case this electronic synchronization might go wrong.
Furthermore, it is known to convert a continuous rotational motion into a reciprocating motion by using motion transmissions. Its most simple embodiment is consisting of a crank or an eccentric, driving a connecting rod, such that it may perform a harmonic motion. In many cases however, a harmonic motion is not the course of motion desired for the reciprocating motion because when motion is reversed, no real standstill will occur and the speed is continuously varying. In many applications, realizing an important standstill is an important advantage when reversing a motion, for instance, enabling a rapier to move through the shed when the shed is formed. The warp yarns should take up a position creating a minimal opening in order to enable the rapier to extend through the shed. With a harmonic course of motion of the heddle frames, driving the warp yarns, the rapiers will only be moving through the shed when the weaving frame has formed a sufficiently large shed, but the weaving frame will continue to move during the period the rapiers are moving through the shed and will form a larger shed which is not required for the application. In other applications it is an essential advantage to obtain a speed which is as constant as possible when moving, for instance, when cutting through a face-to-face fabric. All along the width of the face-to-face fabric, it is desirable to cut the sandwich fabric at a speed which is as constant as possible in order to obtain a uniformly cut pile surface. Therefore, as an alternative to the crank or the eccentric, cams are used, on which a cam follower will follow the cam profile, whereas a spring, for instance, is pushing the cam follower against the cam. In some cases, this negative control will lead to an uncertainty with respect to the position, because of the reaction time of the spring and the possible vibrations of the spring which might occur. Therefore, with rapidly rotating applications, cams are used consisting of two complementary cam discs in combination with a cam follower, provided with contact rollers which will follow each form of the cam disc. A contact roller is moving about each disc, such that the cam follower is rocking as a function of the combination of the profiles of the two complementary cam discs and its rocking motion is transferred to a driven component. Also three-dimensional cams, such as globoidal cams are used in combination with a cam follower to convert a continuous rotational motion into a rocking motion.
Another possibility consists in linking a continuously rotating shaft with a modulator in order to obtain an irregularly rotating motion of the output shaft which may be further converted into a reciprocating motion, functioning in accordance with a corrected or modulated course of motion.
As far as modulators are concerned, it is essentially known to use them in co-operation with electronic dobby devices to drive the heddle frames in a weaving machine.
In EP 446 155 a modulator is described, in which the complementary cam is fixedly connected to the housing and a rotating cage of a known shape is installed around this complementary cam and rotating in connection with the continuously rotating driving shaft. On each of the two continuous shafts situated in the cage, diametrically opposite one another, a cam follower is mounted on bearings, its contact rollers following the cam profiles of the complementary cam. Each of these cam followers drives a connecting rod, which is linked to a support on the driven shaft. The rocking motion of the cam followers is transmitted to the output shaft, by means of the connecting rods, giving cause to decelerations and accelerations on the output shaft with respect to the continuously rotating driving shaft. By accurately dimensioning the cam it may be obtained that in certain places the driven shaft will temporarily be brought to a standstill. By using complementary cams it will be possible with these devices to determine very accurately the course of the motion. However, the cage, rotating together with the drive shaft, has the disadvantage that it will extend about the conjugated cams, which means that an important mass, is well away from the axis of rotation, and will be rotating. In this solution the mass which is centrally situated is not moving and the mass situated around it is indeed rotating. This is a great disadvantage as far as inertia is concerned, which will cause more and more problems as operating speeds of these devices will increase. The rocking levers also are situated on the outside of the stationary cams and also their mass in motion will give cause to additional inertia. Such an embodiment likewise means that either the drive shaft or the driven shaft should extend through the fixed cam, which will increase the length of the construction of the modulator, without this part of the shaft being an active contribution to the transmission. Since the complementary cam is centrally situated and each cam follower, following the conjugated cams, has two contact rollers each of which being in contact with one cam profile of the conjugated cams, it is not possible to insert another rotating part in the radial direction in the cross-section in which a cam profile with contact roller is situated. This is a restriction on obtaining compact embodiments.
In addition to this, in the solution described above, both the contact rollers of the cam followers and the support on the driven shaft are subjected to an overhanging load. This will strongly increase the load on the bearings, causing untimely wear. This will likewise cause vibrations which will have harmful consequences as endurance is concerned and will cause restrictions as to the permissible speeds of the device.
In FR 2 478 143, to which EP 46 155 is referring for known embodiments of rotating cages, the solutions are also based on a fixed cam about which cam followers are moving, which are linked to a support, rotating together with the drive shaft. In the more specific embodiments, a cage is described rotating about the fixed cam. The rocking levers are provided with a slot in which a little sliding block is moving, which is hingedly attached to two flanges. These flanges constitute a support which is mounted on the driven shaft and which extends through the fixed cams.
This solution has the same disadvantages with respect to inertia and compactness as EP 446 155, since also here, the stationary fixed cam is centrally installed with respect to the drive shaft.
U.S. Pat. No. 4,924,915 describes an embodiment of a modulator which also has a fixed cam about which a rotating cage is installed, one of its flanges being externally provided with a gearwheel, meshing with a gearwheel on the continuously rotating drive shaft, not being coaxial with the driven shaft.
This embodiment is intended to limit the length of the device, but it is particularly capacious in the cross direction and which is still having the same disadvantages as inertia is concerned.
In addition, also an embodiment is known of a dobby device, which is used for face-to-face velvet machines of the Velvet Tronic type, a cage is rotating about a fixed cam, attached to the drive shaft. In the cage, two cam followers are hingedly incorporated, each being provided with a toothed segment meshing with a gearwheel on the output shaft, such that the rocking motion of the cam followers is superposed onto the continuously rotating motion of the drive shaft to obtain an irregularly rotating motion on the output shaft.
Here also the same disadvantages are prevailing as to inertia and compactness, as already referred to above. Additional disadvantages of this embodiment are the cost price of the expensive and difficult to produce toothed segments and the disadvantage of a gearwheel drive, such as developing heat and wear.